This invention relates generally to the condensation of metal vapours, and to the continuous or semi-continuous withdrawal of liquid metal from a condenser.
The invention is described hereinafter with particular reference to the condensation of magnesium vapour, and the withdrawal of liquid magnesium from a condenser. It is to be understood that this is given by way of a non-limiting example and that the principles of the invention can be applied to other volatile metals such as zinc, calcium, sodium, potassium, and phosphorus.
The metallic vapour may be pure or it may be mixed with an inert gas such as argon, and it may contain dross, i.e. solid process contaminants, such as magnesium oxide.
U.S. Pat. No. 2,971,833 entitled “Process of Manufacturing Magnesium” (J. Artru et al) describes what is known as a Magnetherm furnace and condenser for the manufacture of magnesium. The Magnetherm process involves the thermal production of magnesium vapour in a furnace and the condensation of the vapour in a condenser. The vapour is conducted to a condensation zone in the condenser where it is condensed as partly liquid and partly solid magnesium. The process is carried out under a vacuum of 2 kPa to 10 kPa and the condenser crucible is cooled from the outside either by water-spray cooling or by immersion in a tank with circulating water. Since the furnace-condenser system is maintained under vacuum, and because the magnesium in the condenser is mostly solid, the process is essentially a batch process which is repeated every 12 to 24 hours. The vacuum has to be broken to remove the slag periodically from the furnace and to replace the full condenser crucible with an empty one.
The product vapour generated in a thermal magnesium process such as the Magnetherm process inherently contains some undesirable solid particles of magnesium oxide, magnesium nitride, silicon oxide, calcium oxide, aluminium oxide, and carbon. The mixture of these solid particles is called dross. Most of the dross particles accumulate in the condenser crucible. In order to achieve continuous magnesium operation, a means of dross removal from the condenser has to be included in the design and the operating method of the condenser. The magnesium generating furnace may contain an underflow-overflow weir arrangement for continuous withdrawal of slag and residual ferrosilicon which are produced as by-products in the furnace to make the whole process of magnesium generation and condensation continuous.
It is an objective of this invention to provide a method and apparatus for continuous metal condensation and withdrawal of condensed liquid metal by operating at, or close to, atmospheric pressure, by suitable temperature control of the condenser, and by stirring of the condenser contents to allow continuous or semi-continuous withdrawal of metal from a condenser crucible.